Solution mining a stable roof under an inert gas

ABSTRACT

This method includes a solution mined underground salt cavern, wherein the salt cavern has a main body with a mean diameter of D N , and an upper portion comprising an inert gas pad, a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching an Nth tier adjacent to the upper portion having a height H 1  and a mean diameter D N+1  that is smaller than D N  by a ratio R raising the inert gas pad by an amount A 1 , providing a stream of leaching water which is injected below the inert gas pad with a velocity V, thereby leaching a N+1th tier adjacent to the Nth tier having a height H 2  and a to a mean diameter D N+2  that is smaller than D N+1  by a ratio R, and repeating steps c and d a number of times T, thereby forming a stable roof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/711,182 filed May 13, 2015 which claims the benefit of priority under35 U.S.C. §119 (a) and (b) to U.S. Provisional Patent Application No.62/089,564 filed Dec. 9, 2014, the entire contents of which areincorporated herein by reference.

BACKGROUND

Leached storage caverns in salt formations typically have a relativelyflat roof. Large diameter flat roofs in a salt cavern can be unstabledue to the low tensile strength of the salt, salt movement, fracturedsalt or low pressure in the cavern. The stability of the roof may beincreased by leaching a modified dome shape in the roof.

The leaching of storage caverns in salt formations is typicallyperformed under a blanket of liquid hydrocarbons. Some storageapplications may require very clean or ultra pure caverns, whereresidual hydrocarbons could contaminate the stored product. To preventthese contamination issues, ultra pure salt caverns can be leached underan inert gas blanket. This invention claims that to increase the overallstability of the roof of a leached salt cavern, a domed roof is leachedusing in an inert gas blanket

SUMMARY

This method includes a solution mined underground salt cavern, whereinthe salt cavern has a main body with a mean diameter of D_(N), and anupper portion comprising an inert gas pad, a stream of leaching waterwhich is injected below the inert gas pad with a velocity V, therebyleaching an Nth tier adjacent to the upper portion having a height H1and a mean diameter D_(N+1) that is smaller than D_(N) by a ratio Rraising the inert gas pad by an amount A1, providing a stream ofleaching water which is injected below the inert gas pad with a velocityV, thereby leaching a N+1th tier adjacent to the Nth tier having aheight H2 and a to a mean diameter D_(N+2) that is smaller than D_(N+1)by a ratio R, and repeating steps c and d a number of times T, therebyforming a stable roof.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates an embodiment of the invention;

FIG. 2 illustrates an embodiment of the invention;

FIG. 3 illustrates an embodiment of the invention;

FIG. 4 illustrates an embodiment of the invention;

FIG. 5 illustrates an embodiment of the invention; and

FIG. 6 illustrates an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below. While theinvention is susceptible to various modifications and alternative forms,specific embodiments thereof have been shown by way of example in thedrawings and are herein described in detail. It should be understood,however, that the description herein of specific embodiments is notintended to limit the invention to the particular forms disclosed, buton the contrary, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

It will of course be appreciated that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

This invention claims that to increase the overall stability of the roofof a leached salt cavern, a domed roof 204 is leached using in an inertgas blanket 102, which could be nitrogen, helium, argon or methane. Theinert gas blanket is injected into the outer annulus string 106 of asalt cavern 101. The blanket pressure is maintained at a pressure abovethe water injection pressure but less than the maximum pressure for thecavern as defined by the depth of the final cemented casing shoe and themaximum pressure gradient for the cavern.

The blanket depth may be controlled by monitoring the blanket gaspressure and by verification of the blanket depth may be by wire linedensity measurement for the gas-brine interface. Inert gas depth may beraised in increments of between 20 to 40 ft at a time. The cavern roofmay be leached to a diameter 20 to 30 percent less than the oldessentially flat roof. Once the new roof is leached to the smallerdiameter, the inert gas blanket is raised another 20 to 40 ft and thecavern roof is leached to a diameter 20 to 30 percent less than the oldroof. This process continues until the final geometry of the cavernapproximates a dome. Geometry of the roof is verified by through pipesonar.

It is further claimed that the geometry of the storage cavern roof maybe controlled by the flow of water 103 into the cavern. The waterinjection flow into the cavern may be maintained between the minimumflow rate of 5 ft/sec velocity and the maximum flow rate of 8 ft/sec.Ideal cavern roof geometry is achieved by flowing at a constant flowrate of approximately between 6 and 7 ft/sec.

Turning to FIG. 1, a solution mined underground salt cavern 101 has amain body with a mean diameter of D₁, and an upper portion comprising aninert gas pad 102. The inert gas is selected from the group consistingof nitrogen, helium, argon, or methane. A stream of leaching water 103is injected below inert gas pad 102 with a velocity V. Velocity V may bebetween 5 feet/second and 9 feet per second, preferably V may be between6 feet/second and 7 feet/second.

The inert gas pad has a pressure, and a depth defined by the interfacebetween the inert gas and a brine/water mixture produced by the solutionmining, and the inert gas pad depth may be determined by monitoring thegas pad pressure. The inert gas pad depth may be verified by wire linedensity measurement at the interface.

Now turning to FIG. 2, inert gas pad height is raised H1 feet. H1 may bebetween 20 feet and 40 feet, preferably between 25 feet and 35 feet,more preferably 30 feet. As leaching water 103 is injected, it nowreaches region 104, which had previously been protected from leaching byinert gas pad 102, thereby solution mining this portion of the roof ofcavern 101.

Turning to FIG. 3, leaching water 103 produces a first tier 201 in thetop of cavern 101. First tier 201 has a height H1 above the nominal roofof the cavern R, and a mean diameter D₂ that is smaller than D₁ by apredetermined ratio R. The ratio R bay be between 15% and 35%,preferably it may be between 20% and 30%, even more preferably it may be25%.

Now turning to FIG. 4, inert gas pad height is raised H2 feet. H2 may bebetween 10 feet and 50 feet, preferably between 20 feet and 40 feet,preferably between 25 feet and 35 feet, more preferably 30 feet. Asleaching water 103 is injected, it now reaches region 105, which hadpreviously been protected from leaching by inert gas pad 102, therebysolution mining this portion of the roof of cavern 101.

Turning to FIG. 5, leaching water 103 produces a second tier 202 in thetop of cavern 101. First tier 202 has a height H2 above first tier 201,and a mean diameter D₃ that is smaller than D₂ by a predetermined ratioR. The ratio R bay be between 15% and 35%, preferably it may be between20% and 30%, even more preferably it may be 25%.

As indicated in FIG. 6, a stream of leaching water which is injectedbelow the inert gas pad with a velocity V leaches a N+1th tier adjacentto the Nth tier having a height H2 and a to a mean diameter D_(N+2) thatis smaller than D_(N+1) by a predetermined ratio R. These steps arerepeated a predetermined number of times T, thereby forming a stable,dome shaped roof.

1: A method of solution mining a stable roof under an inert gas blanket,comprising: a) providing a solution mined underground salt cavern,wherein said salt cavern has a main body with a mean diameter of D_(N),and an upper portion comprising an inert gas pad, b) providing a streamof leaching water which is injected below the inert gas pad with avelocity V, thereby leaching an Nth tier adjacent to the upper portionhaving a height H1 and a mean diameter D_(N+1) that is smaller thanD_(N) by a predetermined ratio of between 15 and 35%, c) raising theinert gas pad by a predetermined amount A1, d) providing a stream ofleaching water which is injected below the inert gas pad, therebyleaching a N+1th tier adjacent to the Nth tier having a height H2 and ato a mean diameter D_(N+2) that is smaller than D_(N+1) by apredetermined ratio R, e) repeating steps c and d a predetermined numberof times T, thereby forming a stable roof. 2: The method of solutionmining a stable roof under an inert gas blanket of claim 1, wherein theinert gas is selected from the group consisting of nitrogen, helium,argon, or methane. 3: The method of solution mining a stable roof underan inert gas blanket of claim 1, wherein the inert gas is nitrogen. 4:The method of solution mining a stable roof under an inert gas blanketof claim 1, wherein H2 is between 10 and 50 feet. 5: The method ofsolution mining a stable roof under an inert gas blanket of claim 1,wherein H2 is between 20 and 40 feet. 6: The method of solution mining astable roof under an inert gas blanket of claim 1, wherein H2 is 30feet. 7-11. (canceled) 12: The method of solution mining a stable roofunder an inert gas blanket of claim 1, wherein the inert gas pad has apressure, and a depth defined by the interface between the inert gas anda brine/water mixture produced by the solution mining, and wherein theinert gas pad depth is determined by monitoring the gas pad pressure.13: The method of solution mining a stable roof under an inert gasblanket of claim 1, wherein the inert gas pad has a depth defined by theinterface between the inert gas and a brine/water mixture produced bythe solution mining, and wherein the inert gas pad depth is verified bywire line density measurement at the interface.